iOS 11.4.1 update

Dear Team,

I tried to install iOS 11.4.1 update in my iPhone 6s Plus, i tried both WiFi and iTunes method, when using WiFi the phone showing a msg "error occured". when i tried using iTunes, it's stated that server couldn't establish secure connection.

I dont face any problem previously, i trust apple products but i don't know what happened? Please advise me that steps should i measure, or what can i do. thanks